Dr. Mann appeared Monday at news conference organized by a state representative, Greg Vitali, to promote two bills that would provide subsidies for solar installations and increase the amount of electricity that the state’s electric power utilities must generate from renewable sources of energy.

Appearing with Representative Vitali, Dr. Mann criticized those who would deny climate change and mankind’s responsibility for it, even in the face of recent weather extremes
“One often hears the misleading claim that no single event, regardless of how extreme or unprecedented, can be blamed on climate change,” he said. “That is like saying that no single roll of a ‘six’ with loaded dice can be blamed on the loading of the dice.

“Just as the unusual number of ‘sixes’ rolled is due to the loading of the dice, so are the more frequent and extreme weather events we are seeing due collectively to the ‘loading’ of weather dice by increased heat-trapping greenhouse gases produced by our continued burning of fossil fuels,” he told reporters.

Representative Vitali, a Democrat, called on state lawmakers in the Republican-majority legislature to pass his bills, which would establish $25 million a year in financing for rebates for residential, commercial and nonprofit solar installations and raise Pennsylvania’s alternative energy portfolio standard to 15 percent by 2023 from the current 8 percent.

Requiring greater use of renewable fuels would help bring Pennsylvania closer into line with neighbors like New Jersey and Delaware, which have higher requirements for use of renewables, said Bruce Burcat, executive director of the Mid-Atlantic Renewable Energy Coalition.

Representative Vitali said that Pennsylvania contributed 1 percent of the world’s greenhouse gas emissions and accused Gov. Tom Corbett, a Republican, of failing to “even acknowledge the climate crisis.”

Patrick Henderson, Governor Corbett’s energy executive, said that the lawmaker was “simply wrong” in accusing the governor of avoiding public statements on climate change. He cited the governor’s role in introducing a solar farm in Phoenixville, Pa., at a ceremony last April.

He said that the governor, a strong supporter of the state’s booming natural gas industry, had also backed a multistate initiative to convert government vehicle fleets from gasoline to natural gas.

By encouraging the production of natural gas from the gas-rich Marcellus Shale, Mr. Henderson said, Pennsylvania is doing its bit to cut carbon dioxide emissions, which dropped to a 20-year low nationally last year, in part because of the increased use of natural gas by electricity generators rather than coal.

“Put simply, one of the best things to happen in addressing climate change emissions is the emergence of the Marcellus Shale and other natural gas plays in the United States,” Mr. Henderson wrote in an e-mail.

“This market-based approach is much more desirable” than Representative Vitali’s “approach of mandating and subsidizing select energy sources,” he said.

Yet another state lawmaker, Representative Brendan Boyle, a Democrat, said recent national data showing that 2012 was the warmest year on record should be a “wake-up call” for action on climate change.

Efforts to reduce carbon emissions don’t have to be made at the expense of economic growth, he added. “We don’t have to choose between economic growth and dealing with global climate change,” he said.

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How are climate change, scarcer resources, population growth and other challenges reshaping society? From science to business to politics to living, our reporters track the high-stakes pursuit of a greener globe in a dialogue with experts and readers.